Sooo... I'm guessing that QR Hub motors are not recommended?

[Ch00paKabrA]

I have been on this forum for over a year and never saw these discussed. Are they a bad idea from years ago? Or have they escaped notice until now?



How would you use a torque arm?

 

[amberwolf]

I'm guessing they are probably such low power / torque that a torque arm may not be strictly necessary, but without a link or specs or name to research, I couldnt' say for sure.


Keep in mind that typically a hollow axle isn't as strong as a solid one, so you probably wouldn't want to plan on high jumps with that. I am curious about the wire exit, if it is thru the axle, too, or if it is out of a larger bearing's ID area instead.

 

[Ch00paKabrA]

I saw one on aliexpress and searched around to look up on it. This is what I found:

They are 36v 250w small hub motors and the wires do not exit through the axle and they exit on the right side.

http://www.uumotor.com/quick-release-hub-motor-250w-front-wheel-drive.html

uumotor blog post from almost 2 years ago (so they have been around):

http://www.uumotor.com/blog/2012/12/quick-release-electric-bike-conversion-kit/

This one has some decent pictures and it is less than $140.00:

http://www.aliexpress.com/item/36V-...or-E-bike-Quick-Release-Motor/1889184134.html

 

[dogman dan]

My guess would be that you'd use one on a bike with dropouts that are torque arms, or use them with torque arms or good torque washers.

It's quite possible to get a decent amount of pressure with a QR. With a good beefy steel front fork dropout, why wouldn't it work? Might be sketchy on bikes with dropouts not designed for a motor.

+1 on the wire exit design. Clearly they put a larger size bearing on that side, and have lots of room for the wire to come out, away from the axle.

Do that on all hubmotors, with an integrated torque arm, and any size or power hubmotor could be QR, or just a nut with a round axle. It's not all of the design I've been wanting to see for years, but it's a start.

I want flat side axles GONE.

 

[Modbikemax]

Keep in mind that typically a hollow axle isn't as strong as a solid one

Really? I would have thought the opposite is the case. I expect it comes down to cost. Nuts and a solid axle are cheaper.
It is typical Chineese manufacturing 95% very good and 5% shit. Unfortunately it's the 5% that lets the side down.
They could machine a nice spline on the end of the axle and fit a very strong torque arm but near enough is good enough.
The brake levers on a motorcycle brake drum are a good example of how the torque arm should look.

 

[Ch00paKabrA]

It would negate the need for C washers. I'll have to give this some consideration. Thanks for the input.

To my earlier question; I guess they just escaped notice then.

 

[amberwolf]

Keep in mind that typically a hollow axle isn't as strong as a solid one

Really? I would have thought the opposite is the case.

I'm going by things like this:
http://www.cyclingforums.com/t/366034/strength-of-solid-versus-hollow-axles

Recently therehave been a few posts which may confuse (or indeed
originate from) uneducated reader(s) concerning the strength of solid
versus hollow axles.

The strength of any particular cross-sectional shape can be described
by quantity called the section modulus, which is kind of a combination
of the amount of material and the distance away from the bending line
- usually taken as the centre - of the section.

If we take a typical rear axle the OD is pretty close to 0.4 inch. A
hollow axle has an ID of pretty close to 0.2 inch - giving a tube or
column with a wall thickness of almost 0.1 inch.

Using these numbers:

The solid axle will have a section modulus of 0.00627.
The hollow axle will have a section modulus of 0.00588

For front axles, which are around 0.35 OD, the numbers are:

Solid - 0.00435
Hollow - 0.00390

Now, of course, these figures mean that a solid axle is stronger.
There are other factors which affect the strength of axles, of course;
the choice of material, the heat treatment, the surface finish,
etcetera.

A particular effect which cannot be eliminated from hollow axles is
that of the roughness of the inside surface of the hole. Solid axles,
of course, do not have this, and so also do not suffer from the chance
that a crack may start at an irregularity on the inner surface. Cycle
axles of the highest prices (and, the buyer may sometimes presume, the
highest quality) are finished very well on the outside surface; but
the inside is a hidden quantity, difficult to finish and inspect.

In the real world of cycling, there are axles made from different
material and to different qualities. Of course it is possible to find
a particular hollow axle which may be stronger than another particular
solid axle - but to claim in abstract that a hollow axle is stronger
(or even no weaker than) than a solid one is clearly wrong.

It's not a lot of difference, probably, but there's some.

Also, since we are talking about other forces (twisting) on these motor axles than just the ones on regular bike axles, I expect there is a little more difference between them, but I woudln't have a clue to the math to find out.

I *would* say that I've seen more threads about axles that broke on the hollow side rather than the solid side, but even that isn't necessarily statistically significant, since I have no idea what percentage of people total with broken axles have even posted.


Anyway, the QR solves at least one problem with threaded/flatted axles, whether solid or not: Tight nuts.

Now that the little hamster in my brain has the squeaky wheel spinning, some other thoughts:

With the QR, it's a lot easier to get the right tension to tighten things up correctly than it is with nuts, and they are a lot less likely in my experience to work their way loose, at least from typical bicycle forces.

Since I expect there are a lot more *dropout* failures from twisting axles than there are from axles that actually broke from being hollow, it probably means QR axles, hollow or not, could be a much better solution even for higher powered motors.

As long as you don't tighten things so far that you snap the QR skewer from tension, it ought to work ok.



Now, another thought:
http://www.bikeforums.net/bicycle-mechanics/228399-solid-axle-stronger.html#post3053106

Someone brings up another point:

With a solid axle, the tightening of the axle nuts causes the axle to stretch slightly, whereas with a QR axle, the tightening of the QR causes the dropouts to press against the locknuts, causing the axle to compress slightly. Apparently, tensioning the axle reduces the strength of the axle a bit, while compressing it doesn't have that effect...

which is partly countered here:
http://www.bikeforums.net/bicycle-mechanics/228399-solid-axle-stronger.html#post3053440

A solid axle does not actually mean that the axle is under tension when the bolts are tightened down. The only part that's under tension is the part of the axle between the locknut and the bolt - basically across the width of the frame's rear dropout, and then a little bit extra on each side as the force on the locknuts and bolts is distributed over more than one thread. The compression/tension on most of the length of a solid axle is effectively neutral.
A q/r hollow axle is under compression, however, when the q/r is tightened. This has ramifications for adjusting the bearings in q/r vs. solid-axle hubs, because the q/r axle compresses when the q/r is tightened, and thus tightens against the bearings, so must be adjusted with some play left in the bearings when the axle is not under compression.
Given that a solid axle is not under compression or tension, and a q/r axle is under compression, I'm dubious as to whether this makes the q/r hollow axle stronger.

However, it is true that a solid axle isn't much stronger than a hollow axle on the basis of its being solid vs. hollow. A rod's resistance to bending is proportional (and maybe quadratically proportional) to the diameter of the rod, so basically the material further from the center of the rod matters a lot more than the material at the center of the rod. So, a rod's being hollow (and missing material at the center of the rod) doesn't make much difference.

That discussion continues off and on thru the rest of the thread.

 

[d8veh]

For low power light weight bikes, I'm sure they'll b e fine. I don't see the hollow axle as a problem because most motors have an axle that's hollow on one side anyway. Personally, I'd be a bit nervous about whether the QR skewer can get enough compression to provide the friction to counter the torque. I guess it's fully tested, but something in my head says it might slip. You could add a torque arm, but then it's not QR anymore, so no point in having it.

I think I might give one a try if I wanted a lightweight folding bike with small wheels, where the torque on the axle is lower, but not on a normal bicycle.

 

[dogman dan]

Again, clearly the intent is to run it on a bike with beefy dropouts. Then the frame is your TA.

 

[Chalo]

With a nutted axle, friction between the nut face and frame, and between the nut and axle thread, constitutes part of the torque retention system. A QR axle doesn't offer that, so the axle flats and dropout slot only get whatever help they can from the axle locknut face (or what would be a locknut in the case of a normal bike hub). Such surface should be serrated for grip, but in this case does not appear to be.

If only manufacturers would furnish decent integrated torque arms, like those of a coaster brake or drum brake hub, then it would not matter how we fastened the axle to the frame. But they can't do that. They have to save a nickel even if it costs us our bike frames, hub motor wiring, or serious injury.

 

[Modbikemax]

When I look at a standard hub motor I see a lot of compromises. The biggest one is it has been made to fit into a standard bicycle drop out. If you were to build an ebike from scratch, motor and all, you would do things differently for sure but then the motor probably would not retro fit into an existing bike. They guys who make these things want the biggest ready made market to sell into so they make them fit standard bicycles and as we know it just isn't quite right.

My guess is the thing would have a single sided swing arm with the stator attached to the swing arm, ala BMW, and the wheel and rotor would come off the other side.

 

[amberwolf]

The heinzmann motor with it's built-in torque arm will fit about as many standard bikes as a typical flatted-axle hubmotor, so it's certainly possible to "do it right".

It's just that it costs more, is more complicated to manufacture, and typical applications of low-power motors don't require the torque arm--a torque washer or little torque arm that basically increases the axle flat area contacting something is usually enough.

If axle spinout was a really common problem in those applications (which are the majority of the market), manufacturers, at least some of them, would have done something about it by now, out of necessity.

Our typical of application on ES is a little higher power/torque than the world average so we see a lot more axle spinouts.


It'd still be nice if someone other than heinzmann offered a motor with the integrated arm.

 

[Ch00paKabrA]

Again, clearly the intent is to run it on a bike with beefy dropouts. Then the frame is your TA.

One of the things I hear repeated quite often is that most forks are insufficient and need torque arms. Since this this design won't work in a rear motor (freewheel on one side and rotor on the other) because there is no way to get the wires out.

So since it has to be a front and fronts most likely need torque arms, would that eliminate this as a vialble option or am I missing something?

 

[Chalo]

Since this this design won't work in a rear motor (freewheel on one side and rotor on the other) because there is no way to get the wires out.

The vast majority of bicycles don't have, or need, bolted-on brake rotors.

 

[Gregory]

It would be nice and easy to steal a QR hub motor. You'd need to ensure you lock a chain through the front wheel every time.